Basically, test equipment for the testing of circuit boards may be divided into two categories, namely finger testers and parallel testers. Parallel testers are test devices that, by means of an adapter, simultaneously contact all or at least the majority of contact points of a circuit board to be tested. Finger testers are test devices for the testing of non-componented or componented circuit boards, which scan the individual contact points sequentially with two or more test fingers.
In the testing of non-componented circuit boards, as compared with the testing of componented circuit boards or in-circuit testing, many more circuit board test points must be contacted. Consequently the main criterion for successful marketing of a finger tester for non-componented circuit boards is the throughput of contacted circuit board test points within a predetermined period of time.
The test fingers are generally fastened to a slide that is able to move along cross-bars, while the cross-bars in turn are guided and able to move along guide rails. The slides may thus be positioned at any desired point on a test field, which is usually rectangular. For contacting a contact point of a circuit board to be tested, the slide is able to move vertically on the cross-bar, so that the test finger may be placed on the contact point of the circuit board from above and from below.
A finger tester is described in EP 0 468 153 A1, and a method of testing circuit boards using a finger tester is described in EP 0 853 242 A1. EP 990 912 A discloses a test probe for a finger tester in which a test needle is movably guided in such a way that it can extend out from the test probe to contact a circuit board test point. If a circuit board test point is contacted, then the test needle can deflect to the side, thereby limiting the mechanical stress on the circuit board test point. In this case the test needle is driven by an electromagnetic drive. Also known are test probes in which a sprung test needle is used. If the test needle is mounted vertically relative to the circuit board to be tested there is the disadvantage that two circuit board test points lying closely adjacent to one another can not be contacted since, owing to the size of the sprung test needles, the latter cannot be arranged with their probe tips as close together as may be desired.
To avoid this disadvantage, the test needles have been mounted—in a suitable tester—at an angle to the circuit board to be tested. This makes it possible to arrange two test needles with their probe tips very close together. However this has the drawback that, in pressing down the sprung test needles, the probe tip is moved along the surface of the circuit board to be tested. With high contact speeds this can cause a scratch on the circuit board. Moreover, due to the inclined position of the test needle, the point at which the circuit board is contacted is imprecise, since the probe tip is moved parallel to the surface of the circuit board.
To avoid these problems, test probes have been developed with a relatively long, horizontally mounted spring arm, with the test needle formed on its end. On this long spring arm it is advantageous that a deflection by only a small angle produces a relatively large spring movement. By this means the movement parallel to the surface of the circuit board to be tested may be kept small but not completely avoided. With a test probe of this kind there is also a risk of the surface of a circuit board to be tested being scratched. In addition the size of the spring arm makes the test probe relatively heavy, which can lead to damage to the circuit board if the probe tip is placed on it at high speed. To reduce such damage a photoelectric switch, which can detect any deflection of the spring arm, is provided in the area of the spring arm. In the event of deflection of the spring arm, the movement of the test probe is braked or stopped, so that any further damage to the circuit board is avoided as much as possible.
Owing to the considerable size of the spring arm it is very costly to shield it from electrical radiation, which is problematic in the case of measurements with high-frequency signals.
Another known test probe has, as test needle, a rigid needle, which is fastened to a mount by means of a parallel steering unit. The parallel steering unit is comprised of two retaining arms made of plastic, with one end attached to a mount, while the rigid needle is attached to the other end of the retaining arms. When the parallel steering unit is swivelled, this needle may be moved vertically upwards. An end section of the test needle carrying the probe tip is bent relative to the remainder of the test needle, so that the probe tip projects a little from the test probe. By this means it is possible for two closely adjacent circuit board test points to be contacted by two test probes. The retaining arms of the parallel steering unit are so dimensioned that a swivelling movement of the parallel steering unit produces the smallest possible movement parallel to the surface of the circuit board.
The disadvantage of this test probe is that the cable used to supply the measuring signal and which is attached to the test needle, due to its strength and weight, generates a considerable impulse when the test probe impinges rapidly on a circuit board, which may lead to damage of the circuit board to be tested. This applies especially to an embodiment in which two test needles are attached to a parallel steering unit, with a cable attached to each test needle so that a 4-wire measurement may be made.
Disclosed by U.S. Pat. No. 5,804,982 is a test probe for testing the contact points of integrated circuits. This test probe has two elastic retaining arms, which are fastened by one end to a frame of a tester. The two retaining arms are arranged parallel to one another and have, at the end remote from the frame, a non-magnetic body, which is mounted between the two end points of the retaining arms. Fitted in the lower section of the body is a test needle. Provided between the retaining arms are magnetic coils which act together with a further magnet in such a way that they are able to exert a vertically downwards force on the retaining arms.
A contact point of an integrated circuit is contacted by means of this test probe by excitation of the magnetic coils located in the test probe, so that the test needle is moved on to the contact point.
WO 96/24069 concerns a device for the testing of flat components (in-circuit test). The test probe of this device has a pivotable test needle which with one end contacts the test point of the flat component, and at the other end has a movement mimic arrangement to swivel the test needle.